Contents

Alternative names

A number of other names have been used in the past, or in other areas including: Canada Thistle,[4] Canadian Thistle, Lettuce From Hell Thistle, California Thistle,[5] Corn Thistle, Cursed Thistle, Field Thistle, Green Thistle, Hard Thistle, Perennial Thistle, Prickly Thistle, Small-flowered Thistle and Way Thistle. The first two names are in wide use in the United States, despite being a misleading designation (it is not of Canadian origin).[6]

Stems are green smooth and glabrous (having no Trichome or glaucousness), mostly without spiny wings. The stems often lie partly flat by summer but can stay erect if supported by other vegetation. The leaves are very spiny, lobed, up to 15–20 cm long and 2–3 cm broad (smaller on the upper part of the flower stem).

The inflorescence is 10–22 mm diameter, pink-purple, with all the florets of similar form (no division into disc and ray florets). The flowers are usually dioecious, but not invariably so, with some plants bearing hermaphrodite flowers. The seeds are 4–5 mm long, with a feathery pappus which assists in wind dispersal.[3][7][8] The plant also spreads underground using rhizomes.

C. arvense is a C3 carbon fixation plant.[11] The C3 plants, originated during Mesozoic and Paleozoic eras, and tend to thrive in areas where sunlight intensity is moderate, temperatures are moderate, and ground water is plentiful. C3 plants lose 97% of the water taken up through their roots to transpiration.[12]

Status as a weed

The species is widely considered a weed even where it is native, for example being designated an "injurious weed" in the United Kingdom under the Weeds Act 1959.[18] It is also a serious invasive species in many additional regions where it has been introduced, usually accidentally as a contaminant in cereal crop seeds. It is cited as a noxious weed in several countries; for example Australia, Brazil, Canada, Ireland, New Zealand, and the United States. Many countries regulate this plant, or its parts (i.e., seed) as a contaminant of other imported products such as grains for consumption or seeds for propagation. In Canada, Cirsium arvense is classified as a primary noxious weed seed in the Weed Seeds Order 2005 which applies to Canada's Seeds Regulations.[19]

Control methods include:

cutting at flower stem extension before the flower buds open to prevent seed spread. Repeated cutting at the same growth stage over several years may "wear down" the plant.

Orellia ruficauda feeds on Canada thistle has been reported to be the most effective biological control agent for that plant.[20] Its larvae parasitize the seed heads of the plant feeding solely upon fertile seed heads.[21]

Aceria anthocoptes feeds on this species and is considered to be a good potential biological control agent.

Uses

Electron scan micrography of Aceria anthocoptes, a potential biological control agent of the weed, Canada

Like other Cirsium species, the roots are edible, though rarely used, not least because of their propensity to induce flatulence in some people. The taproot is considered the most nutritious. The leaves are also edible, though the spines make their preparation for food too tedious to be worthwhile. The stalks, however, are also edible and more easily de-spined.[23]